1. Field of the Invention
The present invention is directed to a method of generating or modifying a pattern of topically specific magnetic properties in a planar magnetic surface and to magnetic storage devices comprising one or more surfaces provided with such patterns.
2. Discussion of the Prior Art
Magnetic devices for recording and storing electronic information have come a long way from magnetic tapes to current storage devices, such as state-of-the-art hard disks with capacities in the Gigabyte range.
Generally, increases of storage capacity in the past were due, at least in part, to an increase of the storage density, e.g. as expressed in terms of bits per square centimeter (b/cm2). For example, the first magnetic disc introduced by IBM in 1956 had a storage density of about 300 b/cm2 (102); a first xe2x80x9cmagicxe2x80x9d threshold of 100 000 b/cm2 (105) was reached about twenty years later. Currently, storage densities of hard disks and other commercially available planar magnetic media are in the range of hundred million b/cm2 (108) so that even an xe2x80x9centry levelxe2x80x9d hard disc of an every-day computer has a storage capacity of several Gigabytes.
While it is expected that technological progress will generally continue at such rates, physical limits of the storage density of planar magnetic media can be predicted because of the finite size of the smallest magnetic entities, or xe2x80x9cdomainsxe2x80x9d, and the limits of clear discrimination between the unit areas, i.e. the capability of clearly distinguishing between one stored bit and its immediate neighbors. In fact, an upper limit of the order of 10 Gigabits/cm2 (1010) could be expected because of the xe2x80x9cparamagnetic limitxe2x80x9d yet increases of storage density in redesigned planar magnetic devices of several orders of magnitude seem possible.
In an attempt to increase data storage density in magnetic recording, Chappert, C. et al (Science, 280 (1998), 1919-1922) in a report about planar patterned media (this report being incorporated herein for all purposes by way of reference,) have proposed to produce magnetic patterns by impacting cobalt-platinum (Coxe2x80x94Pt) sandwich structures and multilayers with ion irradiation, i.e. impacting with atomic particles, through a lithographic resist mask for magnetic patterning without significant modification of roughness and optical properties of the sandwich or multilayer structures. It has been argued that bombardment (this term being used synonymously with xe2x80x9cirradiationxe2x80x9d and including irradiation with confined or xe2x80x9cnarrowxe2x80x9d beams as well as with broad beams) with such atomic particles changes the Pt concentration profile at each Co layer interface which suggests the possibility of locally changing magnetic properties by a technique similar in effect to that of semiconductor doping, i.e. by irradiation through a lithographically defined mask. A simple sandwich structure was irradiated at a fluence or dose of 1016 ions/cm2with 30 keV He+ ions, i.e. a bombardment of sufficient intensity to make the irradiated area in the particular sample paramagnetic at room temperature.
The authors concluded that magnetic patterning induced by bombardment with such atomic particles would allow to create adjoining regions with differing magnetic properties, such as perpendicular versus in-plane magnetization, or paramagnetic, or stripe domain structures in an otherwise optically smooth film.
However, since both applicability as well as effectiveness of bombardment with atomic particles, such as ions, for magnetic patterning may be limited, it is a main object of the present invention to extend the possibilities of generating or modifying patterns of topically specific magnetic orientation in a planar magnetic surface, or film, and to provide improved magnetic storage and recording devices comprising one or more surfaces provided with such patterns.
We have found that this object and further improvements can generally be achieved according to the invention by subjecting magnetic materials to the impact of energized subatomic particles, namely electrons, neutrons or photons (X-rays). It has been found that such irradiation is capable of causing irreversible changes of magnetic properties, such as magnetic anisotropies as well as domain pinning behavior.
Further, we have established that bombardment with energized subatomic particles, notably in the form of electron radiation, can be used advantageously to magnetically pattern films by irradiating the material either by a narrow or broad beam of subatomic particles, or by shining such a beam through a layer of a material that is substantially opaque to the radiation used except for openings where the radiation is to impact the layer to be treated according to the invention, e.g. a shadow mask or a patterned resist, such as patterned by lithographic means.
The method according to the invention can be extended to include sequential patterning of multilayer films as they are grown in order to optimize their magnetic properties. In view of considerations of energy exchange properties, it is expected that energized subatomic particles other than electrons, i.e. high-energy photons such as X-ray radiation, as well as energized neutrons, will be of similar utility according to the invention as electron radiation.
Thus, according to a first general aspect, the invention provides for a method of generating or modifying a pattern of topically specific magnetization including but not limited to magnetic spin orientation in a surface of a material, preferably in the form of a thin or ultrathin layer, which material is, at least potentially, ferromagnetic; the method according to the invention comprises the step of subjecting the surface to a controlled impact of energized subatomic particles directed at the surface for producing a predetermined pattern of discrete magnetized areas on the surface.
According to a second general aspect, the invention provides for novel magnetic recording or storage devices, such as hard disks or readers for magnetically coded information comprising at least one magnetic surface provided with a pattern of topically specific modifications in a ferromagnetic surface. The pattern comprises a plurality of magnetic domains obtainable by impacting the surface with energized subatomic particles selected from the group consisting of electrons, photons, and neutrons; in a preferred group of such magnetic devices the discrete magnetized area has a largest planar dimension in the order of 0.1-5 xcexcm.
The following definitions of terms believed to be essential for understanding the invention will be used herein:
The term xe2x80x9ctopically specificxe2x80x9d is intended to refer to a specific site, or plurality of specific sites (also termed xe2x80x9cpatternxe2x80x9d), on and in a surface which may be positioned substantially at the top of an electronic recording and/or storage device structure so as to form an interface with the surrounding atmosphere or with a lubricating outermost layer of the type known per se in the art; alternatively, the site or pattern may be situated in a stack of layers of an electronic recording and/or storage device structure, it being implied that such structures are formed of normally solid materials.
The terms xe2x80x9cmagnetizationxe2x80x9d and xe2x80x9cmagnetic modificationxe2x80x9d are intended to include any type of magnetic orientation, or change of a given magnetic orientation, in a material that is, or can be made, magnetic. A magnetic modification also includes but is not restricted to a change from one magnetic state, e.g. ferromagnetic or ferrimagnetic, into any other magnetic state or nonmagnetic state, such as paramagnetic, and vice versa comprising at least one magnetic surface provided with a pattern of topically specific modifications in a ferromagnetic surface, said pattern comprising a plurality of magnetic domains obtainable by impacting said surface with energized subatomic particles selected from the group consisting of electrons, photons, and neutrons.
For brevity, the term xe2x80x9cferromagneticxe2x80x9d will be used herein to include magnetic sites or surfaces formed of either ferromagnetic or ferrimagnetic materials.
A xe2x80x9ccontrolled impact of energized subatomic particlesxe2x80x9d, or xe2x80x9cradiationxe2x80x9d, according to the invention is intended to refer to controlling essentially all parameters of impacting a surface with energized subatomic particles as specified, i.e. electrons, photons and neutrons, generally in the form of beams of radiation, including control of the source of emission or radiation as well as any changes, or avoidance of changes, between the emitter (i.e. particle generator) or radiation source, and the target, i.e. the site or plurality of sites at the surface; also, this term includes at least some control of the site or sites of impact, e.g. by directional control of particle emission or radiation and/or masking.
A xe2x80x9cpredetermined pattern of discrete magnetized areasxe2x80x9d is intended to refer to a generally regular arrangement of a multiplicity of areas in a specific state of magnetization, such as a plurality of ferromagnetic sites or xe2x80x9cisletsxe2x80x9d distributed in an essentially regular manner within a planar continuum or xe2x80x9cseaxe2x80x9d which is in a magnetically different state, e.g. not ferromagnetic or having a different orientation. In a typical predetermined pattern or arrangement of discrete magnetized areas, a multiplicity of linear or circular bands formed of generally isomorphous, e.g. rectangular, polygonal or circular ferromagnetic, or ferrimagnetic areas or xe2x80x9cspotsxe2x80x9d separated from each other by intermediate areas where the adjoining surface is not ferromagnetic.
The term xe2x80x9cenergized subatomic particlesxe2x80x9d is intended to encompass particles with an atomic mass of not more than 1 and having an energy which may differ for the different types of subatomic particles; for example, a suitable energy range for electrons is in the range of from about 100 eV to about 100 keV with a preferred range between about 0.5 and about 20 keV; a suitable energy range for photons is in the range of from about 20 eV to about 50 keV with a preferred range between about 100 eV and about 5 keV; for neutrons, a suitable energy range is between about 10 meV and about 1 MeV.
It will be noted that the terminology as regards magnetic states as well as materials which normally exist, or are capable of existing, in such states are well known in the art, e.g. Cullity, B. D., Introduction to Magnetic Materials, Addison Wesley, 1972 and do not require specific exemplification.